The indigo dyeing of cellulosic yarns and fabrics, because of the essentially surface nature of such dyeing, creates a fabric subject to considerable and persistent washdown or loss of color during extended use. For nearly two decades customer preference and acceptance, particularly in denim fabrics such as jeans and overalls, has been highly favorable to the so-called washed-down look. Often the product was actually prewashed several times by fabric or garment manufacturers to produce the much-sought "used" look. In some instances even one or more chlorine bleaches were employed to achieve a greater appearance of use. Today the tide of customer and styling preference has swung toward denims more stable to washing, in either home or commercial washing machines. Not only is more washfastness being sought in jeans and overalls, but denim has been promoted to a high-fashion fabric for use in suitings, slacks, dresses, and the like.
Since high washdown is inherent in indigo-dyed fabrics, a need has thus arisen for such fabrics which are more resistant to washing than any heretofore known. The present invention concerns a method whereby the washfastness of indigo-dyed cellulosic fabrics, more particularly denims, is greatly improved.